Control of noise created by gas flow through restrictions in piping systems has become increasingly important as noise levels in manufacturing and other industrial facilities have been subjected to close governmental regulation. In addition, in certain defense applications, i.e. submarines, noise control is a critical design objective. A major source of noise in such situations have been identified as being caused by an aerodynamic phenomenon associated with high velocity flow levels created by a rapid expansion of the gas after passing through a flow restriction, creating localized high velocity flow conditions.
In order to prevent such excessive velocities, tortuous flow path elements have been used in conjunction with valves, etc., to gradually decrease the pressure of the gas so that its velocity remains substantially constant and at a relatively low level. Such tortuous path devices have in the past been provided by relatively expensive machined parts or stacked discs in which labyrinth passages are formed.
While such devices may accomplish the desired control of gas velocities, the cost penalty is relatively high and the flexibility of design is limited in adapting to varying flow conditions which has led to the consideration of porous materials, but in most such materials it is difficult to accurately control pore size to prevent localized conditions of high velocity flow created by the occurrence of relatively small openings.
One such material in which relatively precise control over the pore size may be had is a precision wound material of the sort described in U.S. Pat. Nos. 2,857,657 and 3,123,446. This material is formed by a precision winding operation in which wire ribbon material is wound on a mandrel with successive windings being crossed with respect to each other to create porous layers having openings of precisely controlled size. The layers of windings are subsequently diffusion bonded to provide a unitary structure. This approach provides a material in which porosity may be relatively easily controlled by varying the pitch, the crossover angle of the windings, wire size, etc.
In the above described design, a slight taper in the opening size occurs as windings of the same pitch are wound on an increasingly greater diameter created by the previous layer, but such taper is not nearly great enough to compensate for the change in density of the gas. If the pitch of successive windings is increased to provide such increased area openings, periodic interference patterns between windings are created which form small area openings and blockages, causing localized high flow velocities, at least partially defeating the objectives described. In addition, minimum pore sizes must sometimes be held to prevent blockages by particles suspended in the gas.
Accordingly, it is an object of the present invention to provide a controlled porosity material which is formed by successive windings of wire and in which the winding pitch is varied in successive windings, while the interference patterns described are avoided to provide a porous acoustic element.